in iTunes > preferences > advanced: General; set the location to your external drive. you have other options there as to organising your music into folders, and copying files to the set location when adding new music to the library, which are both optional. Any music you rip/import from an audio CD using iTunes will always go to the set location and isn't affected by the "copy when adding" checkbox.
Then File > "Add to library.." and choose your external as the source. That's it.
One caution: if ever you don't have your external mounted and you open iTunes, it will reset the location to the default (the User/Music folder on your internal)